https://en.wikipedia.org/wiki/Libpng
http://libpng.org/pub/png/libpng.html
Reference implementation.
Manipulates PNG encoding / decoding / metadata.
Part of the LSB: https://refspecs.linuxbase.org/LSB_3.1.1/LSB-Desktop-generic/LSB-Desktop-generic/toclibpng.html So basic.
Library is really small, compiles in a snap. Sweet.